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Section 2. Oxidation numbers. Section 2 Oxidation Numbers. Chapter 7. Oxidation Numbers. The charges on the ions in an ionic compound reflect the electron distribution of the compound.

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oxidation numbers1

Section 2 Oxidation Numbers

Chapter 7

Oxidation Numbers
  • The charges on the ions in an ionic compound reflect the electron distribution of the compound.
  • In order to indicate the general distribution of electrons among the bonded atoms in a molecular compound or a polyatomic ion, oxidation numbers are assigned to the atoms composing the compound or ion.
  • Unlike ionic charges, oxidation numbers do not have an exact physical meaning: rather, they serve as useful “bookkeeping” devices to help keep track of electrons.
assigning oxidation numbers

Section 2 Oxidation Numbers

Chapter 7

Assigning Oxidation Numbers
  • In general when assigning oxidation numbers, shared electrons are assumed to “belong” to the more electronegative atom in each bond.
  • More-specific rules are provided by the following guidelines.
    • The atoms in a pure element have an oxidation number of zero.

examples: all atoms in sodium, Na, oxygen, O2, phosphorus, P4, and sulfur, S8, have oxidation numbers of zero.

assigning oxidation numbers continued

Section 2 Oxidation Numbers

Chapter 7

Assigning Oxidation Numbers, continued
  • The more-electronegative element in a binary compound is assigned a negative number equal to the charge it would have as an anion. Likewise for the less-electronegative element.
  • Fluorine has an oxidation number of –1 in all of its compounds because it is the most electronegative element.
assigning oxidation numbers continued1

Section 2 Oxidation Numbers

Chapter 7

Assigning Oxidation Numbers, continued
  • Oxygen usually has an oxidation number of –2.

Exceptions:

      • In peroxides, such as H2O2, oxygen’s oxidation number is –1.
      • In compounds with fluorine, such as OF2, oxygen’s oxidation number is +2.
  • Hydrogen has an oxidation number of +1 in all compounds containing elements that are more electronegative than it; it has an oxidation number of –1 with metals.
assigning oxidation numbers continued2

Section 2 Oxidation Numbers

Chapter 7

Assigning Oxidation Numbers, continued
  • The algebraic sum of the oxidation numbers of all atoms in a neutral compound is equal to zero.
  • The algebraic sum of the oxidation numbers of all atoms in a polyatomic ion is equal to the charge of the ion.
  • Although rules 1 through 7 apply to covalently bonded atoms, oxidation numbers can also be applied to atoms in ionic compounds similarly.
rules for assigning oxidation numbers

Visual Concepts

Rules for Assigning Oxidation Numbers

Go to Blackboard and view “Oxidation Numbers” movie.

assigning oxidation numbers continued3

Section 2 Oxidation Numbers

Chapter 7

Assigning Oxidation Numbers, continued

Sample Problem E

Assign oxidation numbers to each atom in the following compounds or ions:

a. UF6

b. H2SO4

c.

assigning oxidation numbers continued4

Section 2 Oxidation Numbers

Chapter 7

Assigning Oxidation Numbers, continued

Sample Problem E Solution

a. Place known oxidation numbers above the appropriate elements.

Multiply known oxidation numbers by the appropriate number of atoms and place the totals underneath the corresponding elements.

assigning oxidation numbers continued5

Section 2 Oxidation Numbers

Chapter 7

Assigning Oxidation Numbers, continued

Sample Problem E Solution, continued

The compound UF6 is molecular. The sum of the oxidation numbers must equal zero; therefore, the total of positive oxidation numbers is +6.

Divide the total calculated oxidation number by the appropriate number of atoms. There is only one uranium atom in the molecule, so it must have an oxidation number of +6.

assigning oxidation numbers continued6

Section 2 Oxidation Numbers

Chapter 7

Assigning Oxidation Numbers, continued

Sample Problem E Solution, continued

  • Hydrogen has an oxidation number of +1.

Oxygen has an oxidation number of 2.

The sum of the oxidation numbers must equal zero, and there is only one sulfur atom in each molecule of H2SO4.

Because (+2) + (8) = 6, the oxidation number of each sulfur atom must be +6.

assigning oxidation numbers continued7

Section 2 Oxidation Numbers

Chapter 7

Assigning Oxidation Numbers, continued

Sample Problem E Solution, continued

  • The total of the oxidation numbers should equal the overall charge of the anion, 1.

The oxidation number of a single oxygen atom in the ion is 2.

The total oxidation number due to the three oxygen atoms is 6.

For the chlorate ion to have a 1 charge, chlorine must be assigned an oxidation number of +5.

+5 2

+5 6

using oxidation numbers for formulas and names

Section 2 Oxidation Numbers

Chapter 7

Using Oxidation Numbers for Formulas and Names
  • As shown in the table in the next slide, many nonmetals can have more than one oxidation number.
  • These numbers can sometimes be used in the same manner as ionic charges to determine formulas.
    • example: What is the formula of a binary compound formed between sulfur and oxygen?

From the common +4 and +6 oxidation states of sulfur, you could predict that sulfur might form SO2 or SO3.

Both are known compounds.

using oxidation numbers for formulas and names continued

Section 2 Oxidation Numbers

Chapter 7

Using Oxidation Numbers for Formulas and Names, continued
  • Using oxidation numbers, the Stock system, introduced in the previous section for naming ionic compounds, can be used as an alternative to the prefix system for naming binary molecular compounds.